Assymetry: no load and loading conditions

[bob]

Hi everybody,

 

I just installed a CET4 protection relay on my 250 k W slip ring motor and was surprised to see an assymetry of 7 % under no load condition. Hwever, once loaded the assymetry dropped to around 2%. It is also worth mentioning that the magnetising current of the motor is around 40 % FLC.

How is assymetry related to load current ?

 

Bob

[marke]

Hi Bob

 

An imbalance in the magnetising current is normal and can be quite large. This does not cause any problems. The imbalance will usually reduce as the load is increased provided that the voltage balance is reasonable.

 

The magnetising current imbalance is due to a combination of grain orientation in the stator laminations and minor supply imbalances. The imbalance can usually be reduced by rotating the phases around the stator terminations. There is usually one connection that minimises the imbalance. This does not alter the motor rotation.

 

L1 --> X1

L2 --> Y1

L3 --> Z1

 

L1 --> Y1

L2 --> Z1

L3 --> X1

 

L1 --> Z1

L2 --> X1

L3 --> Y1

 

Best regards,

[bob]

Thks Marke for your valuable comment. However, does an assymetry of 10 % normal for a slip ring motor on no load.

I have set up m relay for an assymetry of 5 % , and I will need to disable the tripping on no load. I think it`s going to be too tricky.

Bob

[marke]

Hi Bob

 

I have seen no load currents with an imbalance of greater than 10%. By rolling the phases, we have reduced this to less than 5%. I would try that if possible.

 

Best regards,

[bob]

Hi Marke,

 

The voltage measured on the input side is 401 , 412 , 410 V respectively showing therefore an imbalance of 2.7 %. Could an imbalance of 2.7 % on the input side cause an imbalance of more than 15 % on current.

Now, could suh an imbalance on oltage destroy the motor winding ?

 

Bob

[marke]

Hi Bob

 

That is quite an imbalance and I am surprised that it is not causing a greater imbalance in the magnetising current!

 

This level of imbalance will cause negative sequence currents to flow in the rotor and could be an issue at full load.

 

I would carefully look at the current imbalance at full load as this could be an issue.

 

This level of imbalance suggests to me that something is wrong somewhere on the supply and I would be trying to get that sorted rather than foll the motor protection.

 

Best regards,

[GGOSS]

A motor manufacturer once advised that for every 1% error in line voltage an error of between 6% and 10% in line current could be expected.

 

Rolling the phases can act to counter the possibility of errors in line voltage adding to errors in motor winding impedance. Needlss to say where these do add, errors in line current will be exagerated.

 

The errors in line voltage you report strongly suggest either overloading or loose connections on at least one phase. Needless to say any loose connections should be rectified as soon as possible to prevent premature failre of equipment. As it may not be possible for you to address the error in line voltage, it would seem 'rolling the phases' is worth a shot. You may be pleasantly suprised with the outcome.

 

Regards,

GGOSS

[bob]

I agree that the imbalance in voltage is high. I have contacted the utility company and the HV voltage on the 6.6 k V/ 400 V transfo is 6.61, 6.48 and 6.46 k V, but once my genset starts generating and exporting power to the grid, the imbalance in current on my motor on load dropped to 2-5 % .

 

Bob